In recent years much attention has focused on the use of photovoltaic cells to convert electromagnetic radiation into electric energy. Of particular interest are the various potential usages of solar energy as the source of electricity.
While various forms of photovoltaic cells (often called solar cells) are known, in general they can be described as semiconductor devices which are capable of converting sunlight directly into electricity. Photovoltaic installations will generally consist of small, individual generating units--the photovoltaic cell.
In their early inception they found numerous uses in electronic and aerospace applications. It is surprising that such a large variety of these cells are available today or in advanced development when one considers that it has only been since about 1972 that serious attention has been given to designing cells for use in anything other than spacecraft. Devices are available with a large range of efficiencies and voltages. Some cells are designed to withstand high solar intensities and high operating temperatures, while others are designed to minimize manufacturing costs.
With photovoltaic cells, the energy in light is transferred to electrons in a semiconductor material when a light photon collides with an atom in the material with enough energy to dislodge an electron from a fixed position in the material (i.e. from the valence band), giving it enough energy to move freely in the material (i.e. into the conduction band). A vacant electron position or "hole" is left behind at the site of this collision. Such holes can move if a neighboring electron leaves its site to fill the former hole site. A current is created if these pairs of electrons and holes (which act as positive charges) are separated by an intrinsic voltage in the cell material.
Creating and controlling this intrinsic voltage is the trick which has made semiconductor electronics possible. The most common technique for producing such a voltage is to create an abrupt discontinuity in the conductivity of the cell material (typically silicon in contemporary solid state components) by adding small amounts of impurities or dopants to the pure material. This is called a homojunction cell. An intrinsic voltage can also be created by joining two dissimilar semiconductor materials creating a heterojunction, or by joining semiconductor to metal (e.g. amorphous silicon to palladium), creating a Schottky barrier junction.
A fundamental limit on the performance of all of these devices results from the fact that: (1) light photons lacking the energy required to lift electrons from the valence to the conduction bands (the band gap energy) cannot contribute to photovoltaic current; and (2) the energy given to electrons which exceeds the minimum excitation threshold cannot be recovered as useful electrical current. Most of the unrecovered photon energy is dissipated by heating the cell.
The bulk of the solar energy reaching the earth's surface falls in the visible spectrum where photon energies vary from 1.8 eV (deep red) to 3 eV (violet). In silicon, only about 1.1 eV is required to produce a photovoltaic electron. Choosing a material with a higher energy threshold results in capturing a larger fraction of the energy in higher energy photons but losing a larger fraction of lower energy photons. The theoretical efficiency peaks at approximately 1.5 eV, but the theoretical efficiency remains within 80% of this maximum for materials with band gap energies between 1 and 2.2 eV.
In the evolution of photovoltaic cells, e.g. since the early '70's, the art has been confronted with various problems affecting the efficiency of the cells, all of which need not be discussed herein.
The present invention relates to one of these problems, namely that the usual photovoltaic cells have a sensitivity curve which is limited in a spectrum zone of longer wavelengths. For instance, it is well established in the art that among existing photovoltaic devices, a single crystal silicon photovoltaic cell (sometimes referred to simply as a "silicon cell") provides the highest conversion efficiency of solar energy radiation into electrical energy. However, the problem with the silicon cell in the past has been its relatively low conversion efficiency of 10 to 15 percent in direct sunlight. One of the reasons for this low conversion efficiency is that the specific spectral energy of solar radiation does not provide a good spectral match with the response of a silicon cell.
In attempts to solve this problem, efforts have focused on the fact that it would be advantageous to make use of spectrum regions other than those of high wavelengths and which are endowed with greater energy, e.g. violet, near ultraviolet and remote ultraviolet. Accordingly, efforts have been made in the past to increase the overall efficiency of solar energy collectors and photovoltaic cells, specifically, by disposing between the source of actinic radiation and the cell a luminescent medium or reagent which will absorb radiation at the shorter wavelengths, e.g. the UV, and emit it at longer wavelengths to which the cell is responsive to conversion to electrical energy.
For example, U.S. Pat. No. 3,912,931 issued to Gravisse et al on Oct. 14, 1975 relates to a photovoltaic device having a large surface for receiving radiation having a coating containing at least one luminescent substance so chosen that the response to spectral excitation of the substance is, on the average, situated lower, on the scale of the wavelengths, than the zone of spectral sensitivity of the photovoltaic cell alone. More specifically, the patent discloses a type of solar cell construction which contains a series of thin luminescent layers of different compositions, which are laid over the surface of the photovoltaic cell, the order of succession and the composition of these layers being selected in such manner that the light energy, in a spectrum zone, falling upon the outermost thin layer is transferred in cascade, through the intermediary of the interposed individual layers, to the spectral sensitivity zone of the photovoltaic cell itself.
As examples of other patents relating to improving the conversion efficiency of solar radiation, mention may be made of the following patents which represent but a cursory search and accordingly should not be taken as a complete survey of the art pertaining thereto.
U.S. Pat. No. 3,426,212 issued to Klaas on Feb. 4, 1969 discloses a radiation converter comprising at least two substantially parallel layers each comprising (a) solid polymeric material that is substantially transparent at least in the near ultraviolet and visible regions of the spectrum, and (b) fluorescent substance, the layers containing different fluorescent substances and disposed in optical relationship and adjacent to each other. One object of the invention is said to be to convert radiant energy into chemical energy or into electrical energy by a process involving the production of fluorescence (stokes and/or anti-stokes), generally in a cascade system involving a plurality of fluorescent materials, and passage of the fluorescence into a chemical system or into a photocell or other radiation converter.
U.S. Pat. No. 3,929,510 issued to Kittl on Dec. 30, 1975 discloses a solar radiation conversion system having a silicon cell and solar radiation conversion means comprising ytterbium oxide and at least one other specified rare earth oxide. A specific object is said to be to provide a system having solar energy conversion means characterized by a band-emission spectrum that provides a good spectral match with the spectral response of a silicon cell.
U.S. Pat. No. 4,140,544 issued to Sill on Feb. 20, 1979 pertains to a luminescent photovoltaic device for converting solar or other light radiation to electrical energy, having a luminescent collector member with divergent upper and lower surfaces. These surfaces are divergent in at least that part or area of the collector where solar radiation enters the collector, the direction of divergence being selected to direct the internal surface angle of reflection of collected light energy toward one or more photovoltaic cells mounted on or coupled to one or more edge surfaces of the collector.
U.S. Pat. No. 4,155,371 issued to Wohlmut et al, on May 22, 1979 has as its objective to provide a new and improved luminescent solar collector which employs a plurality of different types of photovoltaic cells. The luminescent solar collector has a luminescent member with at least two types of photovoltaic cells, each type of cell operating efficiently for the generation of electricity over a wavelength range which is different from the efficient wavelength range of the other types of photovoltaic cells present on the luminescent member, each type of photovoltaic cell carrying intermediate to it and the luminescent member a filter means which allows only light within the efficient wavelength range for that type of photovoltaic cell to pass from the luminescent member into the photovoltaic cell.
U.S. Pat. No. 4,166,919 issued to Carlson on Sept. 4, 1979 discloses an amorphous silicon solar cell with a layer of high index of refraction material or a series of layers having high and low indices of refraction material deposited upon a transparent substrate to reflect light of energies greater than the bandgap energy of the amorphous silicon back into the solar cell and transmit solar radiation having an energy less than the bandgap energy of the amorphous silicon. The patent is directed to the problem caused by absorption of infrared radiation by the solar cell degrading the performance and shortening the useful life of the solar cell.
U.S. Pat. No. 4,190,465 issued to Boling on Feb. 26, 1980 discloses a luminescent solar collector comprising a relatively thick layer with a photocell coupled to a face surface thereof and at least one thin luminescent layer optically coupled to said thick layer, the thick layer having an index of refraction of at least 0.04 more than each of the luminescent layers and being at least 10 times as thick as the sum of all other layers of the collector. The invention is said to be an improvement in luminescent solar collectors and concentrators of a previously disclosed type.
U.S. Pat. No. 4,238,247 issued to Oster on Dec. 9, 1980 is concerned with the problem that photovoltaic cells actually utilize a very small portion of the solar energy incident thereon to produce electricity. One object of the patent is to provide a structure for the simultaneous conversion of solar energy to electricity and to thermal energy. This is said to be accomplished by means of a structure which in one apparatus converts a part of incident solar energy to electrical energy by the use of photocells mounted on a luminescent solar collector of tubular design while another portion of such incident energy is converted to thermal energy.
U.S. Pat. No. 4,395,582 issued to Damsker on July 26, 1983 relates to a combined solar converter which has a photovoltaic cell for converting the energy of solar radiation of a particular range of wavelengths to electricity and which has a thermal heat absorber spaced from the cell which converts solar radiation of longer wavelengths passing from the cell to useful heat. In the discussion of the Background Art, mention is made of the relatively narrow range of sensitivity which effectively converts only a small part of the energy to electricity. Consequently, it has previously been proposed to combine solar cells which are sensitive to different parts of the solar spectrum to increase the effective amount of the solar spectrum converted; or to utilize more of the solar spectrum for conversion purposes by shifting radiation of shorter wavelengths to longer wavelengths in order to come within the particular range of sensitivity for a particular cell.
Recently, luminescent solar concentrators (LSC) that make silicon photovoltaic cells more economical to use have been reported in the literature. Basically, an LSC consists of a sheet of transparent plastic permeated with several fluorescent dyes. The plate uses a combination of dyes to deliver reddish light to photovoltaic cells at its edge. When sunlight shines onto the plate, one group of dye molecules absorbs a much wider range of wavelengths of light than would normally be utilized by the photovoltaic silicon cells. These "donor" dyes transfer the energy to"acceptor" dyes within the plate which re-emit the light at the narrow wavelength band that photovoltaic cells use most efficiently. However, it is believed that the dyes need to be stable for 10-20 years if luminescent collectors are to be practical. (See, for example, "Chemical Week", Aug. 25, 1982, page 66, McGraw-Hill, Inc.)
While not intended to be a complete prior art survey, the above citations will illustrate the various approaches that have been or may be taken to increase the efficiency of photovoltaic devices, particularly silicon solar cells. While progress has been made since the early '70's, there is still a great need for modifying the spectral match of the electromagnetic radiation, e.g. solar energy, to the response of the cell, particularly so that the energy at the lower end of the spectrum, e.g. UV, may be utilized.
It is to the latter problem that this invention is directed.
Thus, broadly speaking, the task of the invention can be said to be to provide luminescent materials which can be employed to absorb radiation at the lower wavelength end of the spectrum which cannot be utilized by the photovoltaic cell, but which then emit light at a longer wavelength to which the cell is responsive for conversion to electrical energy. However, since the solution to this task, broadly speaking, is old, the task may be said to be to provide for this purpose luminescent materials which, because of their properties, are particularly suited for this conversion of radiation, e.g. do not have an overlap between absorption and emission which can lower efficiency, have a strong emission at the desired wavelength, are highly stable, and can be synthesized easily and inexpensively.